This invention relates generally to calcium carbonate for use in papermaking, and related industries, and more particularly to a calcium carbonate having acid resistant properties.
Titanium dioxide and calcined clay have traditionally been utilized as filler materials in the preparation of alkaline to weakly acidic paper in order to improve the optical properties, especially the brightness, of the resultant product. These materials, however, especially titanium dioxide, have the disadvantage of being very expensive, resulting in higher manufacturing costs and an uncompetitively priced paper product.
Calcium carbonate, particularly precipitated calcium carbonate, has been used as a filler material in the making of alkaline paper. Such usage results in a paper with enhanced optical properties, without the expense incurred in using titanium dioxide fillers, resulting in a much less expensive product. Calcium carbonate, however, cannot generally be used as a filler in acidic paper because it has low acid-resistance, causing it to decompose in an acidic environment. Consequently, there has long been a need to develop a calcium carbonate composition which is acid stabilized and resistant to decomposition at low/neutral pH, so that it can be utilized as a filler material in the manufacture of acidic paper, such as groundwood paper, where the use of an alkaline filler would have a negative impact on the final paper properties, and where the pH of the process waters tends to increase, thereby increasing its foaming action.
Paper made from mechanical pulps has been traditionally produced under acidic papermaking conditions because of "fiber alkaline darkening" that occurs as pH rises. This means that there is a reduction in brightness of the paper (brightness reversion) when the pH is raised from acid to alkaline in wood-containing systems. Alkaline darkening will occur to some degree in any wood pulps with significant lignin content. The degree of darkening generally depends factors, such as the particular pulps, the pH, and the water quality. In general, ground calcium carbonate and precipitated calcium carbonate fillers serve as a buffer in the 7.5-9.2 pH range when used in the wet end, which is that portion of the paper machine which includes the headbox, wire part, and first press section. Acid-resistant calcium carbonate compositions thus provide a means for reducing the degree of fiber alkaline darkening and brightness reversion due to their ability to maintain a stabilized pH.
A variety of techniques to modify calcium carbonate to achieve acid resistance and to avoid the aforementioned problems are disclosed in the art. For instance, U.S. Pat. No. 5,043,017 discloses and claims an acid-stable calcium carbonate resistant to degradation in a mildly acidic environment which comprises a mixture of a calcium-chelating agent or a conjugate base, and a weak acid, such that calcium carbonate is coated by, and is in equilibrium with, the calcium-chelating agent or conjugate base and the weak acid. Preferred calcium carbonate compositions contain sodium hexametaphosphate and phosphoric acid. A disadvantage of this technique is that some mills are regulated on the amount of phosphates that can be in their effluents, and therefore, can not afford to have extra phosphates being introduced into their system.
U.S. Pat. No. 5,000,791 discloses the preparation of an acid-resistant coating for calcium carbonate particles. This acid-resistant calcium carbonate is prepared by simultaneously mixing the calcium carbonate with a solution of a zinc compound and a solution of a silica-containing substance which is preferably sodium water glass. The zinc compound, preferably, is zinc chloride or zinc oxide. The addition of the zinc compound and the silica-containing substance is in combination with a strong acid. A disadvantage of this technique is that it uses zinc, which generally is an undesirable metal to have in the whitewaters, product, or effluents, since it does not meet EPA standards.
U.S. Pat. No. 5,164,006 discloses and claims an acid resistant calcium carbonate which is prepared by preparing an aqueous slurry of calcium carbonate, heating the slurry to about 75-80 degree Celsius, slowly adding sodium silicate solution in an about 5-10% by weight, adding gaseous carbon dioxide, cooling the slurry and adding zinc chloride to the slurry to bring the pH to a range of 7.5 to 8.0. This technique has the same disadvantage as the aforesaid U.S. Pat. No. 5,000,791 since zinc in the whitewaters, product, or effluents does not meet EPA standards.
Other techniques to surface treat or coat calcium carbonate to achieve higher acid-resistance are disclosed in U.S. Pat. Nos. 5,531,821; 5,593,488; 5,593,489; 5,599,388; and 5,647,902 and in U.S. patent application 08/546,145 owned by the same assignee as this present application.